The present invention relates, in general, to serpentine type tubular heat exchangers located in the vertical gas passageways of steam generators, and more particularly, to the structure and support of the serpentine oriented tubes to cause the drainage of fluid collected therein when the steam generators are shutdown.
It is common practice in the design of a modern high capacity steam generator to provide heat exchange surfaces in the form of closely spaced serpentine metal tube rows disposed in a vertical passageway through which combustion gases at relatively high velocities are conveyed, with the metal tubes in each row having horizontally extending segments conveying the fluid being heated, and being arranged in spaced parallel rows distributed transversely of the direction of gas flow.
Whenever the steam generator is shutdown, water collects along the horizontally extending segments of the heat exchanger tubes. The retention of water in the tubes after a steam generator has been shutdown, if unremoved, will lead to metal pitting corrosion and eventual tube failure if their walls become too thin as a result of the corrosion. Such an event is very costly since it requires the replacement of the weakened or failed tubes thereby resulting in down time of the steam generator.
Pitting corrosion is a localized form of corrosion by which cavities or holes are produced in a metal. Pitting is commonly observed on surfaces with little or no general corrosion. Pitting corrosion is generally of greater concern than uniform corrosion because it is more difficult to detect and protect against. Corrosion products often cover the pits, making them difficult to identify. Apart from localized loss of thickness at the tube metal surface, corrosion pits can also be harmful by acting as stress risers. Corrosion pits are commonly the starting points for cracking and fatigue.
An extremely corrosive microenvironment typically forms within a corrosion pit that varies considerably from the bulk corrosive environment. This corrosive microenvironment can hasten growth of pits once initially formed. Pitting corrosion can produce pits in a variety of configurations. For example, open pits may be formed, or pits may be covered with a semi-permeable skin comprising corrosive products. Pits can be hemispherical or cup-shaped, flat-walled, or completely irregular in shape. Pits may also reveal the crystal structure of the tube metal. Through-shaped pits may be narrow and deep or shallow and wide. Sideways pits may be sub surface, undercutting, or attack the grain of the metal horizontally.
The corrosion problem is exacerbated at locations where attachment welds are made to the heat exchanger tubing, such as with the supports used to maintain the tubes and tube rows of a heat exchanger tube bank in coplanar spaced-apart and parallel relation. The heat affected zones of the attachment welds are prone to pitting corrosion. Weld attachments are also known to restrict the thermal expansion and contraction of the tubing and thus cause it to distort or sag, such that low spots are formed in the horizontal runs of the tubing. These low spots collect water after the steam generator is shutdown and are susceptible to pitting corrosion.